Dicarboxylic acid production with direct fired off-gas heating

ABSTRACT

The invention provides improved energy content in and shaft power recovery from off-gas from xylene oxidation reactions while at the same time minimizing wastewater treatment cost. More shaft power is produced using off-gas than is required to drive the main air compressor, even with preferred, relatively low oxidation temperatures. Simultaneously, an amount of wastewater greater than byproduct water from oxidation of xylene is kept in vapor form and treated along with off-gas pollutants in a self-sustaining (self-fueling) gas-phase thermal oxidative destruction unit. Optionally, off-gas is combined from multiple xylene oxidation reactors, comprising primary and/or secondary oxidation reactors and forming TPA and/or IPA. Optionally, air compressor condensate and caustic scrubber blowdown are used in a TPA process or as utility water, effectively eliminating normal flow of liquid wastewater effluent from a TPA plant. Optionally, PET off-gas containing the water of PET formation is treated in a shared thermal oxidative destruction unit, effectively eliminating normal flow of liquid wastewater effluent from a combined pX-to-TPA-to-PET plant.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application Ser.No. 61/110,235, filed on Oct. 31, 2008, the disclosure of which isincorporated herein by reference in its entirety.

BACKGROUND

It is known to recover the preponderance of vaporized solvent fromreaction off-gas by passing it through at least one cooling, absorbing,and/or distillation means to produce a liquefied recovered solvent. Itis generally desirable to maximize the recovery from reaction off-gas ofvaporized compounds containing at least one hydrocarbyl bond, hereincalled “hydrocarbyl compounds,” “volatile organic compounds,” and “VOC”.

It is known to use at least one distillation means to remove fromrecovered solvent an amount of excess water, which is coproduced inlarge quantities by the partial oxidation of pX. Various designs areknown for using energy derived from the partial oxidation of pX for atleast a portion of the energy input required to operate a distillationmeans.

The term “water of TPA formation” is defined herein as 0.340 kilogram ofwater per kilogram of commercial purity pX feed. This comes from theintended reaction forming TPA from pX according to the stoichiometry:pX+3 O₂ yields TPA+2H₂O. Notwithstanding that small amounts ofimpurities exist within commercial purity pX and that a small amount ofpX is under-oxidized and/or over-oxidized, modern manufacturingfacilities produce commercial purity pX comprising very low amounts ofimpurities and to convert such feed into crude and/or purified TPA withvery high yields. Preferably the overall yield of TPA solid product,crude and/or purified, is at least about 96, or 97, or 98, or 99 molepercent based on the mass of commercial purity pX feed divided by amolecular weight of 106.16 grams per mole. Preferably, the commercialpurity pX feed comprises at least about 0.990, or 0.995, 0.997, or 0.998mass fraction of pX.

It is also known to recover energy, both thermal energy and mechanicalshaft work, from a portion of off-gas in various combinations along withrecovery of vaporized solvent. One known method for energy recovery isto use at least a portion of off-gas to boil a working fluid, e.g.,water or pentane, to produce a vapor. This vapor is used to transferheat to another user, or the vapor is reduced in pressure through anexpander, typically a turboexpander, to produce shaft work output. Theenergy recovery from a turboexpander can be converted directly tomechanical work, such as driving an air supply compressor or othermoving machinery, or to electrical power by driving a rotatingelectrical generator connected to a power distribution and consumingnetwork.

Another known method for energy recovery is to pass at least a portionof the off-gas comprising dinitrogen through a turboexpander. The energyrecovery from a turboexpander can be converted directly to mechanicalwork, such as driving an air supply compressor or other movingmachinery, or to electrical power by driving a rotating electricalgenerator connected to a power distribution and consuming network.

It is also known to send a significant portion of water in vapor form inthe off-gas to a thermal oxidative destruction means (TOD) whereinnoxious gaseous and VOC pollutants, e.g., carbon monoxide, acetic acid,methyl acetate, para-xylene, and methyl bromide, are converted to moreenvironmentally acceptable effluents, e.g., water vapor and carbondioxide. Certain convention systems disclose expelling “the water ofreaction” in vapor form from a para-xylene oxidation reactor into athermal destruction device for removal of noxious pollutants.

SUMMARY

The inventors have discovered preferred embodiments not contemplated inthe prior art. Embodiments of the present invention can provide agreater amount of shaft work power recovery from reaction off-gas ofcertain oxidation reaction media, whether to electrical power generationor directly to mechanical uses, and/or expelling an amount water vaporeven greater than the water of TPA formation, and/or a self-sustaining(self-fueling) TOD. Certain embodiments of the invention can evenprovide a combined facility for pX-to-TPA-to-PET that produceseffectively no liquid wastewater.

In a preferred embodiment, the invention comprises passing substantiallyall of the oxidation reaction off-gas, including both primary andsecondary oxidation reactor sources with both pX and mX feeds, through ashared solvent recovery distillation system, then through a superheatingstep, and then through a 2-stage turboexpander comprising interstageheating in order to produce a greater amount of shaft-work. Thisconfiguration allows exporting electrical power beyond the consumptionof the process air compressors and process liquid and slurry pumps.Flash steam from condensate in the turboexpander heaters is used inanother portion of the TPA process. After the turboexpander, a portionof the water vapor is condensed from the oxidation reaction off-gas toprovide liquid water for various process uses; and the balance of thewater vapor is left in the off-gas, which is sent to a TOD means.Optionally, direct fuel firing is used to heat off-gas, rather thansteam heating, to provide superheat within a turboexpander. Optionally,the outlet pressure of a turboexpander is reduced by recompressingoff-gas after it has passed though a condenser means and liquidknock-out means.

Furthermore, the following embodiments are preferred for other aspectsof the inventive process:

-   -   It is preferred that enough combustible fuel value is left in        off-gas such that its environmental abatement in a TOD,        preferably a Regenerative Thermal Oxidizer (RTO), is        substantially, more preferably completely, self-heating without        addition of fuels not present in the reaction off-gas. It is        still more preferred that a substantial amount of this        combustible fuel value comes from methyl acetate (MeOAc), a        known byproduct of oxidation of pX in acetic acid. The inventors        have discovered how to keep formation of methyl acetate        sufficiently low such that the considerable capital and        operating cost to isolate the methyl acetate and to recover by        hydrolysis the acetic acid content are not justified when        considered against adding purchased fuel to a RTO.    -   Condensed water is often formed from ambient water vapor in        compression systems providing ambient air to TPA oxidation        reactors, and this water is potentially contaminated with        lubricants and seal fluids. It is preferred that this condensed        ambient water is admitted to TPA process liquids, e.g., as        scrubber water, quench water, reflux water, or is used as        utility water, e.g., as cooling tower makeup water, rather than        being sent directly to a liquid wastewater treatment facility.    -   After removal and/or thermal destruction of VOC in off-gas, many        locales require removal of hydrogen bromide from such treated        off-gas before release to ambient. This scrubbing is often done        by aqueous scrubbing to produce a bromine salt, e.g., using an        aqueous solution of sodium hydroxide and sodium bisulfite to        scrub and to produce sodium bromide. The inventors have        discovered that blowdown water used to control the dissolved        solids content in such scrubber water is advantageously used as        utility water, e.g., cooling tower makeup water, rather than        forming liquid wastewater.    -   A PET process also produces water from PET formation reactions,        and this water is often contaminated with various VOC compounds,        e.g., ethylene glycol, acetaldehyde, and various dioxolanes. It        is preferred that at least a portion of contaminated water from        a PET process is processed in a shared, common facility along        with water of TPA formation from an adjacent TPA facility.        Preferably, said contaminated water from PET formation is either        left in vapor form exiting said PET facility for treatment or it        is converted to a vapor form using at least a portion of thermal        energy from said adjacent TPA facility. More preferably water        from PET formation reactions is processed in a shared, common        TOD along with water of TPA formation.

Alone, or in various combinations, the inventions disclosed herein canprovide a pX-to-TPA facility producing very low, even nil, liquidwastewater requiring environmental treatment per unit of TPA production.Further, the inventions can provide a pX-to-TPA-to-PET facilityproducing very low, even nil, liquid wastewater requiring environmentaltreatment per unit of PET production.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates exemplary embodiments of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The inventions herein can be combined with the disclosures of US20070293699 and US 20060047158 (the entire disclosures of which areincorporated herein by reference) for a preferred primary oxidationreaction medium, process, and means for converting pX to TPA, with apreferred oxidizer reactor being a bubble column reactor. Thesereference disclosures comprise numerous preferred mechanical featuresand process conditions for a primary oxidation, with process conditionsnotably including temperatures and gradients, pressures and gradients,flows, compositions and gradients, agitation, and residence times anddistributions. The usages herein for “oxidizable compound”. “solvent”,“oxidant”, “reaction medium”, and “hydrocarbyl” are according to theabove references.

The inventions herein are more preferred when at least a portion ofoff-gas from a secondary oxidation reaction medium is combined with atleast a portion off-gas from a primary oxidation reaction medium beforeprocessing in a solvent recovery and/or dehydration means. A secondaryreaction medium is one receiving most of its feed of aromatic substratefrom an upstream oxidation reactor which may be a primary oxidationreaction medium and/or another secondary reaction medium. See US20070155985 and US 20070208191 for descriptions of a secondaryoxidization reactor optimized around further reaction of the enteringliquid-phase aromatic substrate, including benefits of operating inselected process ranges comprising temperatures, pressures, flows,compositions, agitation, and residence times and distributions, balancedagainst various costs, notably including over-oxidation of substrate,product, and solvent. Herein, this type of secondary oxidation reactoris referred to as a “post-oxidation reactor.” Also, see US 20070208190and US 20070219393 for descriptions of a secondary oxidization reactoroptimized around further reaction of the entering solid-phase aromaticsubstrate, including benefits of operating in selected process rangescomprising temperatures, pressures, flows, compositions, agitation, andresidence times and distributions, balanced against various costs,notably including over-oxidation of substrate, product, and solvent.Herein, this type of secondary oxidation reactor is referred to as a“digestion reactor”.

When intentionally producing power, especially shaft power, bycombusting (oxidatively burning) compounds essentially comprisinghydrocarbyl bonds (fuels), the temperature of said combusting is oftenpushed as high as mechanically practicable in order to maximize powerrecovery according to known thermodynamic principles. On the other hand,when conducting a catalytic partial oxidation to form a chemicalproduct, the temperature and pressure of reaction medium is ordinarilyset to control the resulting yields, conversions, and product purities.Catalytic oxidations of the present invention are sufficiently rapidthat great care is required to maintain suitable liquid-phaseconcentrations of dissolved dioxygen, and this causes a preference forhigher system pressures to provide higher partial pressures of gas-phasedioxygen.

Despite these general preferences for higher temperature for energyrecovery and for higher pressure for TPA product purity, the inventorshave discovered that it is preferred to operate at least a portion of aprimary oxidation reaction medium with the following moderate pressuresand temperatures, even while recovering an improved amount of shaftpower and even while expelling greater amounts of wastewater in vaporform. It is preferred to operate at least a portion of primary oxidationreaction medium with a pressure of less than about 12, 10, 8, 7 bara. Itis preferred to operate at least a portion of primary oxidation reactionmedium with a pressure of at least about 2, or 3, or 4, or 5 bara. It ispreferred to operate at least a portion of primary oxidation reactionmedium with a temperature of less than about 200, or 190, or 180, or170° C. It is preferred to operate at least a portion of primaryoxidation reaction medium with a temperature of at least about 120, or130, or 140, or 150° C., or 155° C., or 160° C.

The inventors have discovered that it is preferred to generate thegreatest volumes and masses of vapor possible at the exit of off-gasfrom reaction medium while satisfying the energy balance as required toobtain preferred reaction temperatures and pressures. Undesirably, thegeneration of larger amounts of vapor increases the difficulty indisengaging liquids and solids from the off-gas exiting a reactionmedium. Undesirably, such an increase in off-gas enlarges the diametersand volumes of conduits and equipment processing reaction off-gas; thisnotably includes a solvent recovery and/or dehydration means.Undesirably, the oxidation reaction medium of the present inventionproduces an off-gas that is sufficiently corrosive to require unusuallyexpensive materials of construction, often comprising titanium.Undesirably, the increased flow of vapor exiting reaction medium isgreatly attenuated in both mass and volume during processing in asolvent recovery and/or dehydration means, wherein the preponderance ofthe hydrocarbyl portion of solvent is recovered. However, the inventorshave discovered that an increased amount of solvent vapor formed inreaction off-gas can be sustained in part as an increased amount ofvapor, essentially comprising water, exiting a solvent recovery and/ordehydration means and entering an off-gas turboexpander, often providingan overall economic advantage in shaft energy recovery that surprisinglyoutweighs the increases in other operating costs and the increases incapital cost. In an embodiment of the present invention thehydrocarbyl-depleted off-gas produced from solvent recovery columncomprises at least 10, or 15, or 20, or 30, or 35, or 40, or 45, or 50wt % water vapor based on the hydrocarbyl-depleted off-gas stream. Inanother embodiment of the invention the hydrocarbyl-depleted off-gasproduced from the solvent recovery column comprises less than 4, or 3,or 2, or 1 wt % acetic acid based on the hydrocarbyl-depleted off-gasstream.

The vapor compounds in reaction off-gas comprise water vapor plus VOC.Non-condensable gaseous compounds in reaction off-gas comprisedinitrogen, dioxygen, carbon monoxide, carbon dioxide, and dihydrogen.Applying various aspects of the present invention, the inventors havediscovered that it is possible and preferred to operate a pX partialoxidation process with increased amounts of vapor compounds in reactionoff-gas as follows. It is preferred that the vapor compounds in areaction off-gas are at least about 0.67, or 0.72, or 0.75, or 0.77kilograms per kilogram of reaction off-gas. It is preferred that thevapor compounds in a reaction off-gas are at least about 12.4, or 13.2,or 13.8, or 14.2 kilograms per kilogram of pX fed to correspondingoxidation reaction medium.

To achieve such great amounts of vapor in reaction off-gas, theinventors have discovered that it is preferred to suppress greatly theambient losses and intentional heat removal from an oxidation reactionmedium of the present invention across conductive, isolating, boundarysurfaces, despite that such oxidation reaction is highly exothermic andrequires great cooling. It is preferred to insulate such that at leastabout 70, or 90, or 95, or 99 percent of the exposed, ambient surfacearea of vessels and/or conduits containing at least a portion ofoxidation reaction medium are covered with at least about 0.01, or 0.02,or 0.04, or 0.08 meters thickness of insulation material. It ispreferred that thermal energy losses through exposed, ambient surfacearea of conduits and/or vessels containing at least a portion ofoxidation reaction medium are less than about 40, or 20, or 10, or 5watts per kilogram of pX fed to corresponding oxidation reaction medium.It is preferred to limit cooling of at least a portion of oxidationreaction medium by utility cooling fluids, e.g., water and air, throughconductive, isolating, heat-exchange boundary surfaces such that thermalenergy removal is less than about 100, or 10, or 0.1, or 0.01 watts perkilogram of pX fed to corresponding oxidation reaction medium.

To achieve such great amounts of vapor in reaction off-gas, theinventors have furthermore discovered it is preferred that feeds to anoxidation reaction medium are as hot as practicable, again despite thatsuch oxidation reaction is highly exothermic and requires great cooling.It is preferred that oxidant feed to at least one oxidation reactionmedium is compressed air wherein cooling is minimized after exitingfinal stage of compression. It is preferred that at least about 50, or70, or 90, or 99 percent of the mass of said compressed air reaches anoxidation reaction medium with a temperature at least about 60, 70, 80,90° C. It is preferred that at least about 50, or 70, or 90, or 99percent of the mass of said compressed air reaches an oxidation reactionmedium with a temperature of at least about the discharge temperature ofa corresponding air compressor minus 40, or 20, or 10, or 5° C. It ispreferred to insulate such that at least about 50, or 70, or 90, or 95percent of the exposed, ambient surface area of conduits, vessels, andcontrols for delivering said compressed air are covered with at leastabout 0.005, or 0.01, or 0.02, or 0.04 meters thickness of insulation.

It is preferred that solvent is recovered from reaction off-gas in atleast one solvent recovery and/or dehydration means and then returned toan oxidation reaction medium with a temperature that is above ambienttemperature and near the temperature of corresponding reaction medium.That is, it is preferred that hydrocarbyl compounds are condensed fromreaction off-gas, appropriately dehydrated, and returned to reactionmedium without being much cooler than reaction off-gas. More preferably,this hot recovered solvent is provided with limited amounts of thermalenergy input through conductive, isolating, heat-exchange boundarysurfaces. As is disclosed elsewhere herein, this result is achieved byappropriately limiting the amount of thermal energy removed in saidsolvent recovery and/or dehydration means. It is preferred that at leastabout 40, or 60, or 80, or 90 weight percent of said recovered solventis supplied to an oxidation reaction medium with a temperature of lessthan about 200, or 190, or 180, or 170° C., since it is preferred not totransfer thermal energy into recovered solvent at a temperature greaterthan the temperature of the primary oxidation reaction medium. It ispreferred that at least about 40, or 60, or 80, or 90 weight percent ofsaid recovered solvent is supplied to an oxidation reaction medium witha temperature of at least about reaction off-gas temperature minus lessthan about 80, or 40, or 20, or 10° C. It is preferred that at leastabout 40, or 60, or 80, or 90 weight percent of said recovered solventis supplied to an oxidation reaction medium with a temperature of atleast about 60, or 90, or 120, or 140° C. It is preferred that at leastabout 40, or 80, or 90, or 98 percent of the net thermal energy input toa solvent recovery and/or dehydration means comes directly from theentering flow of reaction off-gas without thermal energy transferthrough conductive, isolating, heat-exchange boundary surfaces. It ispreferred that at least about 40, or 60, or 80, or 90 weight percent ofsaid recovered solvent exits a solvent recovery and/or dehydration meanswith a temperature of at least about the temperature of correspondingreaction off-gas minus less than about 80, or 40, or 20, or 10° C. whilebeing processed therein using a thermal energy input through conductive,isolating, heat-exchange boundary surfaces of less than about 100, or30, or 10, or 3 kilocalorie per kilogram of recovered solvent enters acorresponding reaction medium with a thermal energy input throughconductive, isolating, heat-exchange boundary surfaces of less thanabout 100, or 30, or 10, or 3 kilocalories per kilogram of recoveredsolvent. It is preferred to insulate such that at least about 70, or 90,or 95, or 99 percent of the exposed, ambient surface area of vesselsand/or conduits containing at least a portion of recovered solvent arecovered with at least about 0.01, or 0.02, or 0.04, or 0.08 metersthickness of insulation material.

It is preferred that filtrate solvent recovered from filtration andwashing of solid TPA is returned to an oxidation reaction medium withelevated temperature provided by transfer of thermal energy throughconductive, isolating, heat-exchange boundary surfaces. Filtrate solventis solvent from mechanical separation and/or/or washing of solid TPAfrom a slurry. One means for obtaining filtrate solvent is filtrationand washing of TPA slurry by any means known in the filtration art, butall other mechanical separations known in the art are contemplated bythe inventors for producing filtrate solvent; e.g., gravity settling,centrifuges, hydroclones, and the like.

Before returning to an oxidation reaction medium, it is preferred tocool at least about 40, or 60, or 70, or 80 weight percent of saidfiltrate solvent to a temperature of less than about 100, or 80, or 70,or 60° C. This usefully reduces the solubility of TPA in the slurry, andit usefully reduces the corrosivity of the filtrate solvent so that lessexpensive materials of construction may be used for conduits, vessels,pumps, and other equipment and controls comprising the storage andprocessing of filtrate solvent. Suitable materials of construction forsaid cooled filtrate solvent comprise various metals and alloys withmoderate corrosion resistance, such as stainless steels or duplexsteels, as alternatives to titanium and other more expensive, highlycorrosion resistant metals and alloys.

However, it is more preferred that at least about 40, or 60, or 70, or80 weight percent of said filtrate solvent is provided to oxidationreaction medium with an inlet temperature of at least about 60, or 90,or 120, or 140° C. It is preferred to use solar energy, thermal energyfrom off-gas, and/or thermal energy from steam condensing at a pressureof less than about 60, or 20, or 8, or 4 bara to heat about 40, or 60,or 70, or 80 weight percent of said filtrate solvent by at least about10, or 20, or 40, or 60° C. before feeding into an oxidation reactionmedium. It is preferred to transfer this thermal energy into filtratesolvent through conductive, isolating, heat-exchange boundary surfaces.

It is preferred that pX is fed to an oxidation reaction medium withelevated temperature. It is preferred that at least about 40, or 60, or70, or 80 weight percent of said pX feed is provided to a reactionmedium with an inlet temperature of at least about 60, or 90, or 120, or140° C. It is preferred to use solar energy, thermal energy fromoff-gas, and/or thermal energy from steam condensing at a pressure ofless than about 60, or 20, or 8, or 4 bara to heat about 40, or 60, or70, or 80 weight percent of said pX by at least about 10, or 20, or 40,or 60° C. above bulk storage and/or ambient temperature before feedinginto an oxidation reaction medium. It is preferred to transfer thisthermal energy into pX through conductive, isolating, heat-exchangeboundary surfaces

Separately or in combination, the hotter feeding temperatures ofcompressed air, recovered solvent, filtrate solvent, and/or pX requiresupplying increased liquid flow into an oxidation reactor in order tomaintain its energy balance in order to achieve preferred operatingtemperatures and pressures. With hotter feeds, more of the heat ofreaction is removed as latent heat of solvent vaporization, rather thansensible heating of feeds, and an increased amount of liquid solventfeed exits the oxidation reactor as solvent vapor in reaction off-gas.Undesirably, supplying increased amounts of liquid solvent feed requiresmore costly pumps, conduits, and controls along with increased amountsof pumping power.

For compression of ambient air, elevating supply temperatures byomitting an after-cooler often increases the amount of water vaporentering the oxidation process, unless a desiccating means is provideddifferent from cooling. Such added water must eventually be separatedand expelled from the oxidation process along with the water of TPAformation in order to maintain the desired solvent composition.Furthermore, when such added water is eventually expelled, whether asvapor or liquid or solid, some purchased, carbon-containing mass isoften lost coincidentally, and an added wastewater load is eventuallycreated according to prior art. Thus, such additional entering watervapor in compressed ambient air may be viewed as doubly undesirable,creating a potential carbon loss and a wastewater increase.

However, by using inventions disclosed elsewhere herein to expelincreased amounts of water as vapor and to use limited, coincidentamounts of VOC as combustion fuel in a TOD, the inventors havediscovered a net, positive benefit for leaving selected amounts of watervapor in compressed ambient air used for oxidant feed. Accordingly, itis preferred that at least about 70, or 80, or 90, or 95 weight percentof oxidant feed to at least one oxidation reaction medium of the presentinvention comprises at least about 0.01, or 0.03, or 0.04, or 0.05kilogram of water per kilogram of pX fed to corresponding oxidationreaction medium and less than about 0.12, or 0.10, or 0.08, or 0.07kilogram of water per kilogram of pX fed to corresponding oxidationreaction medium.

After exiting an oxidation reaction medium, more preferably a primaryoxidation reaction medium, it is preferred to use at least a portion ofoff-gas to generate an amount of shaft work using one or moreturboexpander means. A turboexpander means, or simply turboexpander, isone or more turboexpander steps staged in series, optionally with one ormore interstage heating means. The off-gas exiting the lowest pressurestage of a turboexpander, prior to further process steps, is referred toherein as turboexpander off-gas. It is preferred to locate at least oneturboexpander step such that it is mechanically linked to at least onecompression step for supply of oxidant from ambient air. Such linkage isconveniently provided by a rotating mechanical shaft and/or gearbox.

In order to maximize shaft power, it is desirable to minimize the lossof pressure and thermal energy from off-gas before entering aturboexpander. However, there are competing demands for consumption ofpressure and temperature energy in order to recover solvent and toremove appropriate amounts of water in a solvent recovery and/ordehydration means. Also, capital cost requirements for a solventrecovery and/or dehydration means increase greatly at the reducedpressures preferred for the outlet of a turboexpander means, for thevolumes of off-gas become exceedingly large.

As disclosed herein, the inventors have discovered combinations offeatures that enable and balance the consumption of pressure andtemperature energy from reaction off-gas in a solvent recovery and/ordehydration means against the recovery of shaft power from off-gas in aturboexpander means. Discoveries and enabling disclosures for apreferred solvent recovery and/or dehydration means are containedelsewhere herein. Before proceeding to them, the preferred aspectspertaining to a turboexpander means are disclosed.

Attention is directed to the preferred pressure ranges pertaining toinlets flows to turboexpander steps. It is preferred that the pressureat the off-gas outlet from a solvent recovery and/or dehydration meansis reduced by less than about 2, or 1, or 0.5, or 0.2 bar staticpressure evaluated from where reaction off-gas is formed near an uppersurface of reaction medium. It is preferred that the frictional flowingpressure loss through an optional heating means providing thermal energyto off-gas between an outlet of a solvent recovery and/or dehydrationmeans and an inlet of a turboexpander is less than about 32,000, or16,000, or 8,000, or 4,000 Pascal. It is preferred that the pressure ofoff-gas at an inlet to a first turboexpander step is reduced by lessthan about 2, or 1, or 0.5, or 0.2 bar static pressure evaluated fromwhere reaction off-gas is formed near an upper surface of reactionmedium. It is preferred that the pressure at the inlet to at least onturboexpander step is at least about 2, or 3, or 4, or 5 bara. It ispreferred that the pressure at the inlet to a first turboexpander stepis less than about 12, or 10, or 8, or 7 bara. It is preferred thatflowing frictional pressure loss in any interstage conduits and processsteps, such as heat exchange means, summed between the inlet to a firststage of turboexpander and the outlet of a last stage is less than about64,000, or 32,000 or 16,000, or 8,000 Pascal.

Although it is desirable to minimize the distance from the off-gas exitfrom solvent recovery and/or dehydration means to the inlet of aturboexpander in order to minimize the loss of thermal energy outthrough insulation and the loss of pressure energy by flowing frictionalloss, the inventors have discovered that it is preferred to locate theoff-gas inlet to a turboexpander within less than about 40, or 30, or20, or 10 meters measured upwards from surrounding grade. This maximizesthe reconversion of the elevation head of off-gas into static pressureat the inlet of the turboexpander, since the elevation of off-gasexiting solvent recovery and/or dehydration means can be greater than 50meters above grade.

For greater recovery of shaft power, it is preferred to minimize theback-pressure on a turboexpander. Reduced back-pressure helps tomaximize shaft power recovery with a turboexpander by maximizing thedecompression ratio and volume of exiting gas. However, turboexpanderoff-gas of the present invention has other competing needs. At the veryleast, pressure must be provided for flow through conduits, controls,and various equipment, often comprising a condensing means and anenvironment treatment means, before release to ambient surroundings.Producing turboexpander off-gas at lower pressures causes considerabledifficulties with designs and capital cost in these downstreamprocesses. A greater pressure for turboexpander off-gas is indicated forease in condensing preferred amounts of water and VOC, especially inthose process designs preferring to condense (substantially) “all” ofthe water vapor in expander off-gas. With lower pressures, condensingappropriate portions of the water vapor and VOC from turboexpanderoff-gas is difficult or impossible to achieve using utility coolingfluids with near-ambient temperatures, and refrigerating utility coolingfluids is undesirable for such large heat duties. Also, the requiredphysical size for a heat exchange means is reduced if more pressure isretained in turboexpander off-gas, owing to improved heat exchangecoefficients, to improved temperature differential with any givenutility cooling fluid supply temperature, and to managing velocities,pressure drop, and flow distribution within said heat exchange means.Even after condensing a majority or even preponderance of water vaporand VOC, lower pressures for turboexpander off-gas continue to meanlarger sizes for further downstream conduits, controls, and equipment.Furthermore, some process designs prefer to use expander off-gas orcondenser off-gas to convey TPA product powder, and this may causeanother need for increased turboexpander back-pressure.

According to one aspect of the present invention, the inventors havediscovered that the disclosed designs for off-gas conduits, controls,heat exchanger means, TOD means, and scrubber means enable the followingpreferred pressure conditions at the outlet of an off-gas turboexpander.It is preferred that the pressure of turboexpander off-gas is less thanabout 0.9, or 0.6, or 0.4, or 0.3 bar gauge. It is preferred that thepressure of turboexpander off-gas is at least about 0.05, or 0.10, or0.15, or 0.20 bar gauge, with this aspect providing enough pressureenergy to flow turboexpander off-gas through disclosed conduits,controls, and equipment and comprising off-gas condenser, condensatedemisting and demisting, TOD, and scrubber, while not comprising are-compression step, before release to ambient surroundings.

According to another aspect of the present invention, it is preferred tominimize further the turboexpander back-pressure by minimizing thedownstream pressure usage as above and by also providing an off-gasrecompression step located after a condenser heat exchange means whereinat least about 10, or 20, or 40, or 80 weight percent of the water vaporpresent in turboexpander off-gas is removed as liquid water. Theinventors have discovered that, even while venting water vapor toambient surroundings according to the inventions herein, efficientremoval of water vapor from off-gas according to inventions hereinenables a recompression step for the remaining off-gas that requiresusefully less power than the increase in power provided by the greaterdecompression in an upstream turboexpander. In addition, it is morepreferable to locate a knock-out means between an off-gas condensermeans and the inlet to a recompression means. (See elsewhere herein fordisclosures and designations for condenser off-gas and knock-outoff-gas.) When using off-gas recompression, it is preferred torecompress condenser off-gas, more preferably knock-out off-gas, by atleast about 0.05, or 0.1, or 0.2, or 0.3 bar. When using off-gasrecompression, it is preferred to recompress condenser off-gas, morepreferably knock-out off-gas, by less than about 0.9, or 0.8, or 0.7, or0.6 bar. When using off-gas recompression, it is preferred that thepressure of off-gas exiting the lowest pressure stage of a turboexpanderis less than about 0.3, or 0.2, or 0.1, or 0.0 bar gauge. When usingoff-gas recompression, it is preferred that the pressure of off-gasexiting the lowest pressure stage of a turboexpander is at least about−0.9, or −0.6, or −0.4, or −0.3 bar gauge. When using off-gasrecompression, it is preferred to locate at least one recompression stepsuch that it is mechanically linked to at least one turboexpander stepand/or at least one compression step for supply of oxidant from ambientair. Such linkage is conveniently provided by a rotating mechanicalshaft and/or gearbox.

Attention is now directed to the preferred temperatures for an off-gasinlet to a turboexpander means or, if optionally provided, at the inletof an off-gas preheating means placed after a solvent recovery and/ordehydration means and before said turboexpander means. It is preferredthat the temperature at the inlet to a first turboexpander step is atleast about 110, or 120, or 130, or 135° C., evaluated before anyoff-gas preheating means optionally placed ahead of a firstturboexpander means. It is preferred that the temperature at the inletto a first turboexpander step is less than about 190, or 175, or 165, or155° C., evaluated before any off-gas preheating means placed ahead of afirst turboexpander means. It is preferred that the temperaturereduction evaluated from where reaction off-gas is formed near an uppersurface of reaction medium to where off-gas enters a first turboexpandermeans, is less than about a 50, or 40, or 30, or 25° C. reduction,evaluated before any off-gas preheating means optionally placed ahead ofa first turboexpander means.

Although condensing turboexpanders operating at or below the dewpoint ofa working fluid are well known in the art, certain constituents inoff-gas of the present invention cause excessive amounts of erosion andcorrosion for many materials of construction when used in aturboexpander operating too near to the dewpoint of off-gas. Corrosiveconstituents are believed to comprise carboxylic acids and/or bromine inconjunction with water and/or dioxygen.

Accordingly, it is preferred to operate with the temperature at theoutlet from at least one stage of a turboexpander of at least about 5,or 10, or 20, or 25° C. above the local dewpoint temperature of theoff-gas. More preferably, these temperature clearances from dewpoint aremaintained at the outlet from all stages of a turboexpander. Suchtemperatures are attained by various means comprising limiting themechanical efficiency of a turboexpander, adding thermal energy to theoff-gas between the exit from a solvent recovery and/or dehydrationmeans and the exit of a turboexpander, and/or limiting the pressurereduction through a turboexpander.

However, once the dewpoint is sufficiently avoided, the inventors havediscovered that it is often undesirable with respect to capital cost andoperating cost to operate the current invention with too much superheatin turboexpander off-gas. Accordingly, it is preferred to operate withthe temperature at the outlet of at least one stage of a turboexpanderand at and off-gas condenser inlet of less than about 150, or 120, or90, or 60° C. above the local dewpoint.

A less efficient turboexpander requires less added thermal energy toensure that the turboexpander outlet temperature stays in a preferreddewpoint range. When less enthalpy is removed from the working fluid andconverted to mechanical power, the exiting temperature from theturboexpander is inherently hotter. Depending on the relative costs ofthermal heat and the costs for electrical power, improving mechanicalefficiency of the turbine may be detrimental or beneficial for optimizedcost. The inventors have discovered that when the unit cost of deliveredthermal energy is less than about 0.3 times the cost of electrical powerexpressed in the same units, then it is preferred to maximize themechanical efficiency of the turboexpander and to use additional thermalenergy input to obtain the desired dewpoint range on the expanderoutlet. This is less efficiency than an electrical generating powercycle may achieve, e.g., at least about 0.5 ratio of mechanical energyoutput to thermal energy input, so the use of thermal energy input tothe off-gas might seem ill-advised compared to the shaft work achieved.However, the dewpoint avoidance issue means that the incremental thermalenergy input can be coupled with improved efficiency in the expanderand/or increased decompression therein to achieve a remarkable overallimprovement in energy recovery. Thus, it is preferred that themechanical efficiency of a turboexpander employed in the presentinvention is at least about 65, or 75, or 80, or 85 percent of themaximum shaft work output possible to achieve by an ideal, isentropicexpansion of the off-gas working fluid.

In order to increase mechanical power output from a turboexpander,especially in respect of maintaining the outlet temperature in apreferred range relative to the dewpoint while using a high efficiencyturboexpander, it is preferred to provide the following amounts ofthermal energy into off-gas between exiting a solvent recovery and/ordehydration means and a entering a turboexpander and/or at an interstageposition in a multi-stage turboexpander: at least about 100, or 200, or300, or 350 watts per kilogram of pX fed to corresponding oxidationreaction medium; less than about 1,000, or 800, or 600, or 500 watts perkilogram of pX fed to corresponding oxidation reaction medium; at leastabout 10, or 20, or 30, or 40 watts per kilogram of turboexpanderoff-gas; less than about 100, or 90, or 80, or 70 watts per kilogram ofturboexpander off-gas; off-gas temperature rise from thermal energyinput at least about 10, or 20, or 40, or 60° C.; and off-gastemperature rise from thermal energy input less than about 250, or 200,or 150, or 100° C.

Such amounts of thermal energy are supplied via heat exchange meanscomprising conductive, isolating, heat-exchange boundary surfaces,preferably comprising various corrosion resistant metals and metalalloys as known in the art. Preferably the thermal energy is supplied bya hot working fluid, more preferably steam condensing to form a portionof liquid water condensate. Furthermore, the inventors disclose that itis preferred to form at least a portion of lower pressure flash steamfrom condensate formed in an off-gas heat exchange means and to use atleast a portion of said flash steam in at least one heat exchange meanselsewhere in a TPA production process, e.g., heating a portion ofxylene, recovered solvent, filtrate solvent, TPA solid, and/or off-gas.

Optionally, such amounts of thermal energy are supplied by oxidizing afuel with dioxygen and directly combining the resulting hot reactionproducts into off-gas. Said hot reaction products are admitted at alocation between exiting a solvent recovery and/or dehydration means andentering a turboexpander and/or at an interstage position in amultistage turboexpander. Preferably said fuel comprises hydrocarbylbonds. More preferably, said fuel comprises an alcohol, acetate, and/orhydrocarbon. Still more preferably, said fuel predominantly comprisesmethanol, ethanol, methane, propane, butane, and/or fuel oil. Mostpreferably said fuel comprises at least about 50, or 70, or 90, or 95weight percent methane.

Preferably a portion of compressed ambient air is provided for oxidizingsaid fuel, since off-gas from a solvent recovery and/or dehydrationmeans is often relatively lean in dioxygen and rich in water vapor. Morepreferably at least about 50, or 70, or 90, or 100 weight percent of thestoichiometric amount of dioxygen is provided from compressed ambientair fed to an oxidation reaction zone for said fuel. The stoichiometricamount of dioxygen is the minimum amount required for full conversion ofsupplied fuel into water and carbon dioxide. Still more preferably lessthan least about 300, or 200, or 150, or 120 weight percent of thestoichiometric amount of dioxygen is provided from compressed ambientair fed to an oxidation reaction zone for said fuel. Preferably, thepeak temperature for oxidizing said fuel is at least about 300, or 400,or 600, or 800° C. Preferably, an oxidation catalyst is not used topromote the oxidizing of at least about 10, or 50, or 80, or 95 weightpercent of said fuel. Preferably, at least about 10, or 50, or 80, or 95weight percent of the VOC in off-gas exiting a solvent recovery and/ordehydration means is not combusted before exiting the last stage of aturboexpander.

Besides increasing temperature and pressure at an inlet to aturboexpander, the inventors have discovered that the disclosures hereinare also preferred for increasing the mass of water vapor reaching theinlet of at least one turboexpander means. These compositions areenabled by the disclosures herein pertaining to design and operation ofa primary oxidation reaction medium, of a solvent recovery and/ordehydration means, and of connecting conduits. It is preferred that thecomposition of off-gas flowing into at least one turboexpander stepcomprises at least about 3.0, or 3.3, or 3.5, or 3.6 kilogram of waterper kilogram of pX fed to corresponding oxidation reaction medium. It ispreferred that the composition of off-gas flowing into at least oneturboexpander step comprises at least about 0.38, or 0.42, or 0.44, or0.46 kilogram of water per kilogram of off-gas at the same location. Itis preferred that the mass flow of off-gas into the inlet of at leastone turboexpander step is at least about 6.9, or 7.3, or 7.6, or 7.8kilogram per kilogram of pX fed to corresponding oxidation reactionmedium.

Attention is now returned to a solvent recovery and/or dehydrationmeans. It is generally desirable to maximize the recovery from reactionoff-gas of vaporized compounds containing at least one hydrocarbyl bond,herein called “volatile organic compounds” and “VOC”. If not recoveredfrom off-gas, these compounds are undesirably released to ambientsurroundings or, more preferably, mostly converted to water vapor andcarbon dioxide in a TOD. Although the TOD effluent is moreenvironmentally benign, the loss of VOC from a solvent recovery and/ordehydration means remains an operating cost.

More specifically, it is generally desirable to limit the losses of pX,acetic acid, and methyl acetate in off-gas entering a TOD. Such lossminimization is influenced by various mechanical methods in a solventrecovery and/or dehydration means, but the separation is ultimatelycontrolled by thermodynamics and by the energy expenditure in thesolvent recovery and/or dehydration means. Generally, greaterexpenditures of energy may provide lower losses of VOC. Such energyexpenditures result in lower temperatures and/or higher reflux ratios ina solvent recovery and/or dehydration means.

However, the inventors have discovered that intentionally increasinglosses of volatile organic compounds above their bare minimum results inimproved overall process economies when integrated with the fuel needsof a TOD and the shaft power recovery of a turboexpander.

Accordingly, it is preferred to control the energy removal and energylosses in at least one solvent recovery and/or dehydration meansprocessing reaction off-gas as disclosed herein. It is preferred thatthe temperature of at least about 40, or 60, or 80, or 90 weight percentof off-gas exiting from a solvent recovery and/or dehydration means isless than about 50, or 40, or 30, or 25° C. reduced evaluated from wherereaction off-gas is formed near an upper surface of reaction medium. Itis preferred to insulate at least about 70, or 90, or 95, or 99 percentof the exposed, ambient surface area of conduits, vessels and controlscomprising a solvent recovery and/or dehydration means with at leastabout 0.01, or 0.02, or 0.04, or 0.08 meters thickness of insulationmaterial, despite that vast amounts of thermal energy are eventuallyreleased to ambient surroundings after a turboexpander. It is preferredthat thermal energy losses through exposed, ambient surface area ofconduits and/or vessels comprising a solvent recovery and/or dehydrationmeans are less than about 40, or 20, or 10, or 5 watts per kilogram ofpX fed to corresponding oxidation reaction medium.

It is preferred to limit thermal energy recovery such that less thanabout 1,000, or 100, or 1, or 0.1 watts of thermal energy per kilogramof pX fed to corresponding oxidation reaction medium is removed fromprocess fluids through conductive, isolating, heat-exchange boundarysurfaces located from where reaction off-gas is formed near an uppersurface of reaction medium and until at least about 80, or 90, or 95, or99 weight percent of the dinitrogen therein has passed through aturboexpanders means. Some designs known for recovery of energy fromreaction off-gas comprise condensing and recovering solvent byextracting thermal energy across conductive, isolating, heat-exchangeboundary surfaces to heat and/or vaporize utility fluids prior tooff-gas passing through a turboexpander. The utility fluids are thenused for generation of shaft power and/or transfer of thermal energy inother steps. Exemplary utility heat transfer and/or cooling fluidscomprise water liquid and/or vapor, light aliphatic hydrocarbon liquidand/or vapor, and/or air.

It is preferred that a solvent recovery and/or dehydration meansoperates without adding an azeotropic separation compound. Exemplaryazeotropic distillation compounds comprise n-butyl acetate and/orn-propyl acetate. It is preferred that a solvent recovery and/ordehydration means operates with net addition of less than about 0.1, or0.01, or 0.001, or 0.0001 kilogram of azeotropic distillation compoundsper kilogram of solvent recovered from reaction off-gas.

It is preferred that a solvent recovery and/or dehydration means of thepresent invention comprises a high efficiency distillation meansprocessing at least about 80, or 90, or 95, or 99 weight percent of thenon-condensable gases and/or dinitrogen present in reaction off-gas. Itis preferred that said distillation means comprises at least about 20,or 25, or 30, or 35 ideal stages of separation. It is preferred that theflowing frictional pressure loss of off-gas through said distillationmeans is less than about 60, or 40, or 20, or 10 kilopascal. It ispreferred that any distillation trays are of a low pressure drop designof less than about 1,200, or 900, or 700, or 500 Pascal per tray,notwithstanding that this undesirably limits the operating turndown ofsuch trays. It is more preferred to use structured packing as is knownin the art, notwithstanding the need for expensive, corrosion resistantmetallurgy and also the potential flammability of some metals comprisingtitanium. It is preferred to construct said distillation means using atleast two different vessel diameters wherein the maximum horizontaldiameter of an upper section is less than about 1.0, or 0.96, or 0.92,or 0.90 times the maximum horizontal diameter that is present through atleast about 4 meters of height in a lower section and is processing atleast about 80, or 90, or 95, or 99 weight percent of the dinitrogen inreaction off-gas.

After exiting a turboexpander, it is preferred that at least a portionof off-gas gas is cooled in at least one heat exchange means, hereincalled an off-gas condenser, thereby producing a liquid, herein calledreflux and essentially comprising water, at least a portion of which isfed into said solvent recovery and/or dehydration means. It is preferredthat the various preferred ranges for temperature, pressure, and/orcomposition at the inlet of an off-gas condenser are the same as at anoutlet of a final stage of a turboexpander. It is preferred that theflowing frictional pressure loss of off-gas is less than about 16, or12, or 8, or 4 kilopascals in said off-gas condenser. When operatingwithout an off-gas recompression step, it is preferred that the off-gaspressure exiting said off-gas condenser is at least about 0.02, or 0.08,or 0.12, or 0.16 bar gauge. When operating without an off-gasrecompression step, it is preferred that the off-gas pressure exitingsaid off-gas condenser is less than about 0.6, or 0.5, or 0.4, or 0.3bar gauge. When operating with an optional off-gas recompression step,it is preferred that the off-gas pressure exiting said off-gas condenseris at least about −0.8, or −0.7, or −0.6, or −0.5 bar gauge. Whenoperating with an optional off-gas recompression step, it is preferredthat the off-gas pressure exiting said off-gas condenser is less thanabout 0.1, or 0.0, or −0.1, or −0.2 bar gauge. It is preferred that theoff-gas temperature exiting said off-gas condenser is at least about 30,or 40, or 50, or 60° C. It is preferred that the off-gas temperatureexiting said off-gas condenser is less than about 110, or 100, or 90, or80° C. It is preferred that the off-gas temperature exiting said off-gascondenser is reduced at least about 10, or 20, or 30, or 35° C. belowturboexpander outlet temperature. It is preferred that the off-gastemperature exiting said off-gas condenser is reduced less than about100, or 80, or 70, or 60° C. below turboexpander outlet temperature. Itis preferred that thermal energy of less than about 3,100, or 2,900, or2,700, or 2,500 watts is removed in said off-gas condenser per kilogramof pX fed to corresponding oxidation reaction medium. It is preferredthat thermal energy of at least about 1,600, or 1,800, or 2,000, or2,100 watts is removed in said off-gas condenser per kilogram of pX fedto corresponding oxidation reaction medium.

Reflux amount and temperature are selected and controlled to maximizethe water vapor entering a turboexpander in balance with minimizing theloss of VOC in the off-gas exiting the condenser. It is preferred thatthe flow of reflux to a solvent recovery and/or dehydration meanscomprises at least about 7.0, or 8.0, or 8.5, or 9.0 kilogram of liquidwater per kilogram of water of TPA formation produced in oxidationreactors served by said solvent recovery and/or dehydration means. It ispreferred that the flow of reflux to a solvent recovery and/ordehydration means comprises less than about 12.0, or 11.0, or 10.5, or10.0 kilogram of liquid water per kilogram of water of TPA formationproduced in oxidation reactors served by said solvent recovery and/ordehydration means. It is preferred that the flow of reflux to a solventrecovery and/or dehydration means comprises at least about 0.70, or0.75, or 0.79, or 0.82 kilogram of liquid water per kilogram of watervapor exiting from a solvent recovery and/or dehydration means. It ispreferred that the flow of reflux to a solvent recovery and/ordehydration means comprises less than about 0.98*, or 0.96, or 0.92, or0.90 kilogram of liquid water per kilogram of water vapor exiting fromsolvent recovery and/or dehydration means. (*When operating withoptional direct firing of fuel, more water mass is formed by combustionof fuel.) It is preferred that the temperature of reflux fed to asolvent recovery and/or dehydration means is at least about 40, or 50,or 55, or 60° C. It is preferred that the temperature of reflux fed to asolvent recovery and/or dehydration means is cooled less than 40, or 30,or 20, or 10° C. below temperature of water vapor leaving condenser inoff-gas.

The inventors note that placing an off-gas condenser at such lowpressure according to the present invention greatly increases the volumeof off-gas at a condenser entry and exit. Unless conduits of unusuallylarge diameter are used, flowing velocities and frictional pressure dropare offensive. Accordingly, it is preferred that off-gas conduitsbetween a turboexpander outlet and an off-gas condenser inlet havediameters of at least about 1.2, or 1.5, or 1.8, or 2.1 meters, whichare quite large for pressure-containing process conduits made of variousexpensive, corrosion resistant metals and metal alloys. To mitigate theconduit diameter and cost, it is preferred that the superficial velocityof off-gas in conduits between a turboexpander outlet and an off-gascondenser inlet is at least about 30, or 40, or 50, or 60 meters persecond. These are unusually fast conduit velocities re erosion,especially for a corrosive process gas near its dew point, and carefulcontrol is required versus the dewpoint. At the outlet of an off-gascondenser, the certain presence of liquid droplets increases thepotential for erosion and corrosion, and it is preferred to limitsuperficial velocities in these conduits to less than about 30, or 25,or 20, or 15 meters per second until entering a knock-out means forliquid removal, as is disclosed elsewhere herein.

The inventors also note that operating an off-gas condenser at lowpressure according to the present invention forces use of a lowerprocess temperature in order to condense the required amount of reflux.The lower process temperature pinches closer to the temperature of thecooling fluid, and the lower process pressure causes a reduced filmcoefficient of heat transfer on the process side. All factors force anincreased area of conductive, isolating, heat-exchange boundarysurfaces, which typically comprise various expensive, corrosionresistant metals and metal alloys.

The design challenges and costs for an off-gas condenser of the presentinvention are still further amplified when expelling preferred amountsof water vapor to ambient surroundings according to some aspects of thepresent invention. Expelling selected amounts of water vapor introducesa requirement to control intentionally the amount of energy removed inan off-gas condenser even when operating with new or un-fouledconductive, isolating, heat-exchange boundary surfaces, with lower massflow throughputs and/or energy duties when producing TPA at reducedproduction rates, and with variable temperatures of cooling medium as isoften the case, e.g., due to diurnal and seasonal ambient changes.

A particular challenge for control of an off-gas condenser means is thatmost cooling tower water systems contain amounts of dissolved solidsthat are greatly concentrated by evaporative cooling with ambient air.When the flow of such cooling water is throttled to control the processtemperature of an off-gas condenser, the temperature of exiting coolingwater rises. If temperature of such cooling water rises too far, some ofthe dissolved solids precipitate. Unfortunately, many highly corrosionresistant metal alloys are rapidly attacked and perforated by pittingcorrosion under such tuberculated deposits. Accordingly, the inventorsdisclose the following preferred embodiments for an off-gas condenseraccording to the present invention. As used herein, “condenser off-gas”comprises off-gas wherein at least a portion has been processed in atleast one off-gas condenser.

It is preferred that said off-gas condenser producing liquid watercomprises air-cooling wherein ambient air is in contact with conductive,isolating, heat-exchange boundary surfaces containing said off-gas. Itis preferred that forced draft fans are used to move air flow across theconductive, isolating, heat-exchange boundary surfaces containing saidoff-gas. It is preferred that fan speed, fan blade pitch, air flowcontrol louvers, and/or other air flow and/or air temperature controlmeans are used to adjust the amount of off-gas cooling in response to atleast one process variable; e.g., temperature and/or pressure ofcondenser off-gas; temperature and/or flow rate of condenser liquid;chemical composition of either condenser off-gas and/or condensate byany on-line measurement, e.g., infrared compositional analysis.

It is more preferred that said off-gas condenser producing liquid watercomprises cooling water in contact with conductive, isolating,heat-exchange boundary surfaces containing said off-gas. It is preferredthat cooling water flow rate, cooling water inlet temperature, and/orcooling water outlet temperature are used to adjust the amount ofoff-gas cooling in response to at least one process variable e.g.,temperature and/or pressure of condenser off-gas; temperature and/orflow rate of condenser liquid; chemical composition of either condenseroff-gas and/or condensate by any on-line measurement, e.g., infraredcompositional analysis. It is preferred that at least a portion ofcooling water exiting said water-cooled heat exchange means has atemperature of at least about 50, or 60, or 70, or 80° C. It ispreferred that said cooling water comprises water cooled by directcontact with ambient air. It is more preferred that said cooling wateris “enclosed loop cooling water”. It is preferred that said enclosedloop cooling water comprises a reduced amount of Total Dissolved Solids(TDS), e.g., de-ionized water or steam condensate. It is preferred thatat least a portion of heat is removed from said enclosed loop coolingwater in a heat exchange means comprising utility cooling water cooledby direct contact with ambient air. It is preferred that at least aportion of heat is removed from said enclosed loop cooling water in aplate-and-frame heat exchange means.

Optionally, it is preferred that at least a portion of the conductive,isolating, heat-exchange boundary surface is removed from duty from timeto time in response to at least one process variable e.g., temperatureand/or pressure of condenser off-gas; temperature and/or flow rate ofcondenser liquid; chemical composition of either condenser off-gasand/or condensate by any on-line measurement, e.g., infraredcompositional analysis. Said surface portion is removed from duty byremoving it from contact with flowing off-gas and/or flowing utilitycooling fluid.

An optional way for controlling the amount of energy removed in anoff-gas condenser is to bypass a portion of turboexpander off-gas aroundsaid condenser, as is disclosed in U.S. Pat. No. 6,504,051, the entiredisclosure of which is incorporated herein by reference. However, suchgas bypassing creates new problems even while solving the need to adjustand control energy removal. Firstly, such bypassing intimately affectsthe mass balance as well as the energy balance because solvent vapor isnot easily condensed from bypassed off-gas. If too much or too littlegas is bypassed, while seeking to satisfy the energy balance, the waterbalance is upset for the solvent recovery system, making the recoveredsolvent become too wet or too dry; and there is also an upset in theamount of VOC sent toward ambient release and/or a TOD. Secondly, it isdesirable to recombine condenser off-gas and bypassed off-gas fortreatment in a shared, common environmental treatment means. However,such recombination is problematic because an aerosol fog is typicallycreated when a colder, liquid-saturated gas flow is combined with awarmer, liquid-saturated gas. Such an aerosol proves dangerous withrespect to pitting corrosion in conduits and equipment, for the aerosolis prone to collect as droplets on cooler and/or less turbulentsurfaces. Rapid removal of such an aerosol from a high velocity processflow is difficult to achieve while limiting pressure drop and/or inputof thermal energy, notwithstanding that such a fog may readily coalesceto rain liquid droplets when provided longer separation times.

Accordingly, the inventors have discovered the following preferredembodiments for the present invention. After exiting a turboexpander, atleast a portion of off-gas gas is bypassed around at least one off-gascondenser to form a “bypassed off-gas” using one or more of thefollowing preferred aspects. It is preferred that said bypassed off-gasis cooled less than about 60, or 50, or 30, or 10° C. in a heat exchangemeans comprising conductive, isolating, heat-exchange boundary surfacesbefore combining with off-gas exiting an off-gas condenser, entering aTOD, and/or being released to ambient surroundings. It is preferred thatsaid bypassed off-gas is at least about 1, or 2, or 4, or 8 weightpercent of all off-gas exiting a turboexpander. It is preferred thatsaid bypassed off-gas is less than about 50, or 40, or 30, or 20 weightpercent of all off-gas exiting a turboexpander. It is preferred that theflow rate of said bypassed off-gas is used to adjust the amount ofoff-gas cooling in response to at least one process variable; e.g.,temperature and/or pressure of condenser off-gas; temperature and/orflow rate of condenser liquid; chemical composition of either condenseroff-gas an/or condensate by any on-line measurement, e.g., infraredcompositional analysis. It is preferred that said bypassed off-gas iscombined with at least a portion of off-gas that has exited an off-gascondenser to form a “mixed off-gas” before release to ambientsurroundings. It is preferred that a “knock-out means” utilizing atleast one of the following features processes at least a portion ofcondenser off-gas, thereby producing a “knock-out off-gas”. Preferably,at least about 10, 50, 98, 99.9 weight percent of the liquid enteringsaid knock-out means is separated and exits comingled with less thanabout 50, or 95, or 99, or 99.8 weight percent of off-gas dinitrogenfrom an opening in the lower 80, or 60, or 40, or 10 percent of theheight of said knock-out means. Preferably, at least a portion of saidknock-out means is located at a lower elevation than at least oneoff-gas condenser providing gas-plus-liquid multiphase flow into saidknock-out means. Preferably, liquid water exits said knock-out meansfrom an opening located below a flow inlet from an off-gas condenser.Preferably, the superficial vertical velocity of off-gas in saidknock-out means is less than about 4, or 3, or 2, or 1 meter per secondat the plane of greatest horizontal diameter. Preferably, thesuperficial horizontal velocity of off-gas in said knock-out means isless than about 6, or 5, or 4, or 3 meters per second at the plane ofgreatest vertical diameter. Preferably, the mean residence time ofoff-gas in said knock-out means is less than about 20, or 13, or 8, or 5seconds. Preferably, the mean residence time of off-gas in saidknock-out means is at least about 0.5, or 1.0, or 1.5, or 2.0 seconds.Preferably, the mean residence time of liquid within said knock-outmeans is at least about 0.5, or 2, or 4, or 8 minutes. Preferably, themean residence time of liquid within said knock-out means is less thanabout 60, or 48, or 24, or 12 minutes. Preferably, at least oneliquid-removing impingement surface, other than pressure isolatingboundary surfaces, is included within said knock-out means. Preferably,the solid surface area in contact with off-gas passing through aknock-out means is at least about 0.0005, or 0.001, or 0.002, or 0.004square meters per kilogram of off-gas exiting said knock-out means.Preferably, at least a portion of off-gas passing through said knock-outmeans contacts at least about 0.001, or 0.005, or 0.01, or 0.02 squaremeters of non-pressure isolating solid surface area per kilogram of pXfed to corresponding oxidation reaction medium. Preferably at leastabout 70, or 80, or 90 percent of liquid droplets smaller than at leastabout 500, or 200, or 75, or 25 microns present in off-gas entering aknock-out means are removed from knock-out off-gas. The inventorsdisclose that these various preferred features for a knock-out means arepreferred in a knock-out means processing condenser off-gas either withor without bypassed off-gas.

It is preferred that at least a portion of bypassed off-gas is processedin a TOD that also processes at least a portion of off-gas that hasexited an off-gas condenser. More preferably, at least a portion ofbypassed off-gas is combined with least a portion of condenser off-gasto form a mixed off-gas before entering a TOD. Most preferably, thermalenergy is added to raise the temperature of said mixed off-gas beforeentering a TOD means. This heating reduces condensation in off-gasconduits, vessels and other enclosures and thus minimizes the cost formaterials of construction. This thermal energy can be added in whole orpart to said mixed off-gas flow, to said bypassed off-gas flow, or tosaid condenser exit off-gas flow. It is preferred that the temperatureof said mixed off-gas is at least about 10, or 20, or 40, or 60° C.above the temperature of the off-gas exiting said off-gas condenser.

The inventors disclose that it is preferred to add thermal energy toknock-out off-gas according to this aspect even without bypassedoff-gas. It is preferred to locate the off-gas condenser and theknock-out means as follows, in order to balance simultaneously the costof pumping cooling water to high elevations, the cost for tallstructures and supports, and the provision of elevation to enablegravity flow of condensed liquid in and/or through an off-gas condenserand/or knock-out means. It is preferred that the elevation of a lowestcooled surface in at least one off-gas condenser is less than about 50,or 30, or 20, or 10 meters above ground surface elevation. It ispreferred that the elevation of a highest cooled surface in at least oneoff-gas condenser is at least about 6, or 9, or 12, or 15 meters aboveground surface elevation. It is preferred that the elevation of a liquidinventory within a knock-out means is at least about 0.5, or 1, or 2, or3 meters above the surface elevation of surround ground. It is preferredthat the elevation of a liquid inventory within a knock-out means isless than about 20, or 15, or 10, or 5 meters above ground surfaceelevation.

In combination with improved power recovery from an off-gasturboexpander and/or self-heating of a TOD according to disclosuresherein, the inventors have discovered a surprising benefit from leavingincreased amounts of water vapor in vented off-gas, notwithstanding thegreater losses of VOC that often accompany such increased amounts ofwater vapor. Accordingly the inventors disclose the following preferredembodiments for “vented water vapor” present in a condenser off-gas, aknock-out off-gas, and/or a TOD inlet off-gas. It is preferred that thevented water vapor is less than about 400, or 300, or 250, or 200 weightpercent of the water of TPA formation. This avoids expending too muchheat of oxidation reaction in vaporizing water liquid in a once throughturboexpander power cycle with subsequent capital and operating costsfor treatment in a TOD and without a concomitant increase in supply ofreflux for a solvent recovery and/or dehydration means. It is preferredthat the vented water vapor is at least about 100, or 110, or 130, or150 weight percent of the water of TPA formation. To avoid overlydehydrating the recovered solvent system and thereby upsetting theoxidation reaction conditions, preferred sources of the water amountexceeding the water of TPA formation are disclosed herein.

It is preferred that vented water vapor in excess of the water of TPAformation comprises at least a portion of water that has entered theprocess with the oxidant supply, more preferably compressed ambient air.The preferred amount of water vapor entering in compressed air isdisclosed elsewhere herein. It is preferred that vented water vapor inexcess of the water of TPA formation comprises at least a portion ofwater formed from over-oxidation of aromatics and solvent. It ispreferred that vented water vapor comprises at least about 0.05, or0.10, or 0.15 kilogram of water formed from over-oxidation of aromaticsand solvent per kilogram of pX fed to corresponding oxidation reactionmedium. It is preferred that vented water vapor comprises less thanabout 0.05, or 0.04, or 0.03 kilogram of water formed fromover-oxidation of aromatics and solvent per kilogram of pX fed tocorresponding oxidation reaction medium.

It is preferred that vented water vapor in excess of the water of TPAformation comprises water formed from injection of pressurized steam (atleast about 50, 90, 95, 99 weight percent water, at least about 110, or140, or 180, or 220° C.) into a process flow comprising liquid solvent.Preferred applications comprise steam as a source of thermal energy andas a flushing medium for conduits, orifices, and equipment. It ispreferred that vented water vapor in excess of the water of TPAformation comprises at least a portion of water used to scrub processvents that subsequently release gas to ambient surroundings. It ispreferred that vented water vapor in excess of the water of TPAformation comprises at least a portion of water used as an azeotropicseparation aid in a solvent purification and/or catalyst recoveryprocess comprising disclosures in U.S. Pat. No. 4,939,297, U.S. Pat. No.7,351,396, and U.S. Pat. App. Pub. No. 2005-0038288.

It is preferred that vented water vapor in excess of the water of TPAformation comprises at least a portion of water formed from oxidation offuel when optionally used to heat off-gas between an outlet from asolvent recovery and/or dehydration means and an outlet from aturboexpander.

It is preferred that vented water vapor in excess of the water of TPAformation comprises at least a portion of water coming from an adjacentPET process and entering the TPA process before the outlet of a TOD,more preferably before the inlet of a TOD. It is preferred that said PETprocess is located such that the minimum horizontal distance from saidTPA process is less than about 1800, 900, 300, 100 meters. It ispreferred that said PET process forms at least a portion of PET usingTPA product from said TPA process. It is preferred that at least aportion of said TPA product is fed into a reaction medium of saidadjacent PET process within less than about 72, or 24, or 12, or 4 hoursafter being formed from para-xylene, para-tolualdehyde and/orpara-toluic acid. It is preferred that the water coming from said PETprocess is at least about 0.02, or 0.2, or 0.3, or 0.4 kilogram perkilogram of water of formation of TPA. It is preferred that the watercoming from said PET process is less than about 1.1, or 0.9, or 0.7, or0.6 kilogram per kilogram of water of formation of TPA.

It is preferred that a TOD is self-fueled by oxidation of off-gascompounds comprising carbon monoxide and VOC, especially by oxidation ofmethyl acetate. It is preferred that the fuel content of off-gas is atleast about 60, 70, 80, 90 percent of all fuel content entering TOD.Fuel content is evaluated as the heat of oxidation reactions yieldingvapor phase products comprising water vapor and carbon dioxide gas. Itis preferred that fuel content of off-gas is less than about 160, or140, or 120, or 110 percent of the minimum fuel content needed tooperate the TOD without a cooling means, e.g., sensible heating of airor other gas/vapor and/or sensible or latent heating of water or otherliquid, whether directly by comingling mass or indirectly acrossisolating conductive heat exchange surfaces.

It is preferred that a TOD is usefully, even predominantly, fueled bymethyl acetate in off-gas according to the following disclosures. Methylacetate is a known byproduct of the liquid-phase oxidation of pX to TPAin acetic acid. Technologies are known in the art for isolating methylacetate and then hydrolyzing it with water to recover acetic acidsolvent and byproduct methanol effluent. The inventors have discoveredthat an efficient TPA synthesis system, e.g., US 20070293699 and US20070208191 (the entire disclosures of which are incorporated byreference herein), provides a useful reduction in the net formation rateof methyl acetate. The CO component in off-gas has a relatively lowheating value, and off-gas often contains relatively small amounts ofMeBr and acetic acid. Methyl acetate provides useful fuel content to aTOD in order to achieve required temperatures and destructionefficiencies of pollutants, including methyl acetate itself. If the fuelcontent of off-gas is too low, then supplementary fuels, e.g., methane,methanol, fuel oil, must be supplied to the TOD in order achieverequired temperatures and destruction efficiencies of pollutants.

The inventors have discovered the following preferred ranges for methylacetate in knock-out off-gas, ranges that usefully balance the fuelvalue of methyl acetate in a TOD against the capital and operating costsfor further suppression of formation of methyl acetate and/or recoveryof its acetic acid content by separation and hydrolysis. It is preferredthat methyl acetate content in knock-out off-gas and/or off-gas enteringa TOD is at least about 0.003, or 0.005, or 0.007, or 0.008 kilogram perkilogram of pX fed to corresponding oxidation reaction medium. It ispreferred that methyl acetate content in knock-out off-gas and/oroff-gas entering a TOD is less than about 0.030, or 0.025, or 0.020, or0.015 kilogram per kilogram of pX fed to corresponding oxidationreaction medium. It is preferred that methyl acetate provides at leastabout 20, or 30, or 40, or 50 percent of all fuel content entering aTOD. In another embodiment of the invention, it is preferred that methylacetate and/or methanol content in knock-out off-gas and/or off-gasentering a TOD is less than about 0.030, or 0.025, or 0.020, or 0.015kilogram per kilogram of pX fed to corresponding oxidation reactionmedium. It is preferred that methyl acetate provides at least about 20,or 30, or 40, or 50 percent of all fuel content entering a TOD.

It is preferred that acetic acid content in knock-out off-gas and/oroff-gas entering a TOD is less than about 0.005, or 0.004, or 0.003, or0.002 kilogram per kilogram of pX fed to corresponding oxidationreaction medium. It is preferred that carbon monoxide content inknock-out off-gas and/or off-gas entering a TOD is less than about 0.45,or 0.40, or 0.35, or 0.30 mole percent evaluated on a dry basis withonly non-condensable gaseous compounds.

However, it is undesirable to waste too much energy, whether combustiblefuel intrinsic in an off-gas or added fuel, in the TOD. Therefore, it ispreferred that the total combustion energy released by a TOD is lessthan 600, or 500, or 450, or 400 kilojoules per kilogram of pX fed tocorresponding oxidation reaction medium. This low amount of requiredcombustion heat is achieved by providing efficient heat integrationbetween hot, treated off-gas near the exit of a TOD and untreatedoff-gas near the entry to said TOD, as is known in the art by variousmeans.

The supply of combustion heat is appropriately controlled with theoperating methods and off-gas compositions as disclosed herein. It ispreferred that a TOD operates with a peak internal temperature of atleast about 200, or 400, or 600, or 800° C. It is preferred that atleast about 94, or 96, or 98, or 99 mole percent of carbons in off-gasentering a TOD are oxidized to CO2 before exiting the TOD. Mostpreferably, the TOD means in all disclosures herein is a RegenerativeThermal Oxidation (RTO) means.

After removal and/or destruction of carbon monoxide and VOC pollutantsin an oxidation reaction off-gas, many locales require substantialremoval of bromine from the treated off-gas. This bromine reduction isoften done by aqueous scrubbing of the treated off-gas from a TOD, e.g.,liquid scrubbing of off-gas using an aqueous solution of sodiumhydroxide and sodium bisulfite to produce sodium bromide salt. Over timethe concentration of various salts builds up in scrubber water, and aneffluent blowdown must be provided along with a makeup of purer water.Preferably, said off-gas scrubber makeup water is filtered water. Morepreferably, said scrubber makeup water is de-mineralized water,de-ionized water, and/or steam condensate.

The inventors have discovered that said scrubber liquid blowdowneffluent essentially comprising water is advantageously used as utilitywater, e.g., cooling tower makeup water. Accordingly, it is preferred touse at least about 0.01, or 0.05 kilogram of off-gas scrubber liquideffluent water per kilogram of pX fed to corresponding oxidationreaction medium as utility water rather than discharging said scrubberwater to a wastewater treatment unit and/or directly to ambientsurroundings. In another embodiment of the invention any effluent waterobtained from a production facility comprising an oxidation reactionmedium can be used at a rate of 0.01 to 0.05 kilogram of effluent waterper kilogram of pX fed corresponding oxidation reaction medium asutility water rather than discharging the utility water to a wastewatertreatment unit and/or directly to ambient surroundings

In an optional and more preferred embodiment, the invention comprisescombining oxidation reaction off-gas, including both primary andsecondary oxidation reactor sources, for processing through a sharedcombined solvent recovery and/or dehydration means, turboexpander means,condenser means, knock-out means, TOD and/or bromine scrubber.

Undesirably with respect to energy recovery, the operating pressures andtemperatures of secondary reaction mediums are often substantiallydifferent, sometimes significantly higher, from a primary reactionmedium and/or from each other. The simple expansion of higher pressurereaction off-gases across a pressure reduction valve into lower pressureoff-gas usually dissipates significant entropy, causing a loss insubsequent ability to produce shaft work. However, the present inventionusefully retains enthalpy in the combined off-gases at the inlet to aturboexpander, and the combined off-gas flows usefully comingle CO andVOC fuel values ahead of a TOD.

It is preferred to process off-gas from at least two distinct reactionmediums in a shared, common solvent recovery and/or dehydration means,turboexpander means, condenser means, knock-out means, TOD and/orbromine scrubber. It is preferred that at least portion of said distinctreaction mediums are separated horizontally from each other by less thanabout 1,000, or 500, or 300, or 150 meters. “Process integrated” meansthat feed or product streams from two different processes/facilities arecombined and processed using at least one piece of common equipmentselected from the group consisting of turboexpander means, condensermeans, knock-out means, TOD and/or bromine scrubber.

When forming an off-gas mixture, it is preferred that the amount of alloff-gas in the mixture that is sourced from secondary oxidation reactionmedium is much less than the amount of all off-gas in the mixture thatis sourced from primary oxidation reaction medium. In mixtures ofprimary and secondary off-gas, it is preferred that off-gas arising fromsecondary oxidation reaction mediums comprises less than about 20, or10, or 5, or 2 weight percent of the mass of combined off-gas and lessthan about 20, or 10, or 5, or 2 weight percent of the mass ofdinitrogen of combined off-gas. In mixtures of primary and secondaryoff-gas, it is preferred that off-gas arising from secondary oxidationreaction mediums comprises at least about 0.1, or 0.2, or 0.4, or 0.8weight percent of the mass of combined of off-gas and less than about0.1, or 0.2, or 0.4, or 0.8 weight percent of the mass of dinitrogen ofcombined off-gas.

It is preferred that at least about 40, or 60, or 80, or 90 weightpercent of reaction off-gas from at least one secondary oxidationreaction medium is combined with at least about 40, or 60, or 80, or 90weight percent of reaction off-gas from a primary oxidation reactionmedium for processing in a shared, common solvent recovery and/ordehydration means, turboexpander means, condenser means, knock-outmeans, TOD, and/or bromine scrubber. It is preferred that at least aportion of off-gas from said secondary medium is formed at a temperatureof at least about 160, or 175, or 190, or 200° C. It is preferred thatat least a portion of off-gas from said secondary medium is formed at atemperature of less than about 250, or 240, or 230, or 220° C. It ispreferred that at least a portion of off-gas from said secondary mediumis formed at a pressure of at least about 7, or 10, or 13, or 16 bara.It is preferred that at least a portion of off-gas from said secondarymedium is formed at a pressure of less than about 40, or 34, or 28, or24 bara.

It is preferred that at least a portion of reaction off-gas from atleast two distinct secondary oxidation reaction mediums are combinedwith each other and with at least a portion of reaction off-gas from aprimary oxidation reaction medium for processing in a shared, commonsolvent recovery and/or dehydration means, turboexpander means,condenser means, knock-out means, TOD, and/or bromine scrubber. It ispreferred that at least a portion of off-gas from at least one secondarymedium is formed at a temperature of less than about 20, or 15, or 10,or 5° C. greater than a portion of said off-gas from said primaryoxidation reaction medium. It is preferred that at least a portion ofoff-gas from one secondary medium is formed at a temperature of at leastabout 10, or 15, or 25, or 35° C. greater than a portion of said off-gasfrom said primary oxidation reaction medium and/or a portion of saidoff-gas from a distinct secondary oxidation reaction medium. It ispreferred that at least a portion of said off-gas from one secondarymedium is formed at a temperature of less than about 20, or 15, or 10,or 5° C. different than a portion of said off-gas from a distinctsecondary oxidation reaction medium. It is preferred that at least oneof said reaction off-gases comes from a secondary oxidation reactionmedium separated horizontally from a portion of said primary reactionmedium by less than about 60, or 20, or 8, or 2 meters. It is preferredthat at least one of said reaction off-gases comes from a secondaryoxidation reaction medium separated horizontally from a portion of saidprimary reaction medium by at least about 4, or 8, or 16, or 32 meters.

It is preferred to process at least about 40, or 60, or 80, or 90 weightpercent of the dinitrogen in reaction off-gas from at least onesecondary oxidation reaction medium combined with at least about 40, or60, or 80, or 90 weight percent of the dinitrogen in reaction off-gasfrom a primary oxidation reaction medium in a shared, commonturboexpander means, condenser means, knock-out means, TOD, and/orbromine scrubber.

The inventions herein are preferred for a process producing a crude TPA,in which total monocarboxylic acid impurities comprise at least about1,000, or 2,000, or 4,000, or 6,000 ppmw; producing a purified TPA, inwhich total monocarboxylic acid impurities comprise less than about1,000, or 500, or 300, or 200 ppmw; and producing both crude andpurified TPA simultaneously in any relative ratio. Monocarboxylic acidimpurities notably comprise benzoic acid, para-toluic acid, and4-carboxybenzaldehyde.

The inventions herein are more preferred for producing purified TPA witha b* color of less than about 4.0, or 3.5, or 3.0, or 2.5 b* units. Theb* value as used herein is one color attribute measured on aspectroscopic instrument such as a Hunter Ultrascan XE instrument(Hunter Associates Laboratory, Inc., 11491 Sunset Hills Road, Reston,Va. 20190-5280, www.hunterlab.com) using a reflectance mode. Positivereadings signify the degree of yellow (or absorbance of blue), whilenegative readings signify the degree of blue (or absorbance of yellow).

The inventions herein apply for processes converting m-xylene (mX) intoisophthalic acid (IPA) in all aspects of the disclosures by substitutingthe meta-species for the para-species, e.g., mX for pX, IPA for TPA,meta-toluic acid for para-toluic acid, and 3-carboxybenzaldehyde for4-carboxybenzaldehyde. This extension applies for a process making IPAseparately. The extension also applies in partially and/or fullycombined processes making both TPA and IPA.

Aspects of the invention pertaining to combining reaction off-gas fromdifferent reaction mediums for processing in at least one solventrecovery and/or dehydration means, turboexpander, condenser, knock-outmeans, TOD, and/or bromine scrubber apply in all aspects also when thepreponderant aromatic species in at least one reaction medium ispara-substituted and the preponderant aromatic species in at least oneother reaction medium is meta-substituted. All aspects of the inventionpertaining to recovered solvent apply also when at least a portion ofsolvent recovered from reaction off-gas from oxidizing one xylene issubsequently used within 72, or 48, or 24, or 12 hours in reactionmedium oxidizing the other xylene. All aspects of the inventionpertaining to filtrate solvent apply also when at least a portion offiltrate solvent formed by separation from a solid product that ispredominantly one aromatic dicarboxylic acid is subsequently used within72, 48, 24, 12 hours in reaction medium oxidizing the other xylene.(Even though it is possible, and often preferable, to keep Co/Mn/Br evenheavy aromatic impurities from one filtrate isolated from the other, itis not preferred to keep the water, acetic acid and benzoic acidcomponents separated.)

It is preferred to treat at least a portion of filtrate solvent(para-filtrate) from a process wherein the preponderant aromatic speciesare para-substituted and a portion of filtrate solvent (meta-filtrate)from a process wherein the preponderant aromatic species aremeta-substituted in a solvent purification and/or catalyst recoveryprocess sharing at least one conduit, pressure containing vessel, and/orrotating equipment item. Suitable filtrate purification units arecomprised disclosures in U.S. Pat. No. 4,939,297, U.S. Pat. No.7,351,396, and U.S. Pat. App. Pub. No. 2005-0038288 (the entiredisclosures of which are incorporated herein by reference). It is morepreferred that at least a portion of para-filtrate and of meta-filtrateis co-fed at the same time, as compared to being campaigned in sequence.It is preferred that at least a portion of purified filtrate and/or atleast a portion of recovered catalyst from said process are subsequentlyused to form at least a portion of oxidization reaction medium andsubsequently to form a portion of para-filtrate or of meta-filtrate,more preferably forming a portion of both filtrates. It is preferredthat at least about 10, or 40, or 80, or 98 weight percent of thearomatic impurities removed from para-filtrate by said filtrate solventpurification and/or catalyst recovery process are physically combinedwith at least about 10, or 40, or 80, or 98 weight percent of thearomatic impurities removed from meta-filtrate by said process.

All aspects of the invention pertaining to compressed air for oxidantsupply apply when a portion of compressed air from a compression meansis divided to provide oxidant supply to at least one oxidation reactionmedium wherein the preponderant aromatic species are para-substitutedand at least one other oxidation reaction medium wherein thepreponderant aromatic species are meta-substituted.

All aspects of the invention pertaining tooxidant-supply-intercooler-condensate apply when at least a portion ofoxidant-supply-intercooler-condensate is divided with part being used inat least one process step wherein the preponderant aromatic species arepara-substituted and with another part being used in at least one otherprocess step wherein the preponderant aromatic species aremeta-substituted.

In one embodiment, it is preferred that that a smaller IPA manufacturingprocess is co-located with a larger TPA manufacturing process. Whenoff-gas from at least one preponderantly meta-substituted reactionmedium is combined with off-gas from at least one preponderantlypara-substituted reaction medium, it is preferred that the off-gasarising from preponderantly meta-substituted medium comprises less thanabout 50, or 40, or 30, or 20 weight percent of the mass of combinedoff-gas and less than about 50, or 40, or 30, or 20 weight percent ofthe mass of dinitrogen in the combined off-gas.

When off-gas from at least one preponderantly meta-substituted reactionmedium is combined with off-gas from at least one preponderantlypara-substituted reaction medium, it is preferred that the off-gasarising from preponderantly meta-substituted mediums comprises at leastabout 0.5, or 1.0, or 1.5, or 2.0 weight percent of the mass of combinedoff-gas and less than about 0.5, or 1.0, or 1.5, or 2.0 weight percentof the mass of dinitrogen in the combined off-gas.

When an ambient air compression means supplies oxidant to at least onepreponderantly meta-substituted reaction medium and to at least onepreponderantly para-substituted reaction medium, it is preferred thatpreponderantly meta-substituted reaction mediums receive less than about50, or 40, or 30, or 20 weight percent of the total mass flow of oxidantfrom said air compression means. When an ambient air compression meanssupplies oxidant to at least one preponderantly meta-substitutedreaction medium and to at least one preponderantly para-substitutedreaction medium, it is preferred that preponderantly meta-substitutedreaction mediums receive at least about 0.5, or 1.0, or 1.5, or 2.0weight percent of the total mass flow of oxidant from said aircompression means.

It is more preferred that said IPA and TPA manufacturing processes areco-located with at least one co-located PET manufacturing process. It ispreferred that IPA produced in said co-located process comprises atleast about 0.5, or 1.0, or 1.5, or 2.0 weight percent of alldicarboxylic acid fed to a co-located PET manufacturing process. It isalso preferred that IPA produced in said co-located process comprisesless than about 16, or 12, or 8, or 4 weight percent of all dicarboxylicacid fed to a co-located PET manufacturing process.

In another embodiment, with or without co-locating IPA and TPA, it morepreferred that at least a portion of IPA fed from a primary reactionmedium comprising preponderantly meta-substituted species is fed to aPET reaction medium without said IPA having been purified by dissolving,selectively hydrotreated, and re-precipitating to remove 3-CBA and/orcolored species.

In one embodiment, it is preferred that said IPA fed to PET reactionmedium is either crude IPA directly from a primary oxidation reactionmedium that is preponderantly meta-substituted or is post-oxidized IPAformed in a secondary reaction medium whose average reaction temperatureis less than about 24, 16, 12, 8 degrees hotter than said primaryreaction medium. It is preferred that said crude IPA and/or postoxidized IPA comprises at least about 20, or 80, or 160, or 320 ppmw of3-CBA; less than about 3,000, or 2,400, or 1,800, or 1,200 ppmw of3-CBA; at least about 2, or 4, or 8, or 16 ppmw of 2,6-DCF; and lessthan about 160, or 120, or 80, or 60 ppmw of 2,6-DCF.

In another embodiment, it is preferred that said IPA fed to PET isdigested IPA formed in a secondary reaction medium whose averagereaction temperature is at least about 16, or 24, or 30, or 36° C.hotter than said primary reaction medium and less than about 80, or 70,or 60, or 50° C. hotter than said primary reaction medium. It ispreferred that said digested IPA comprises at least about 10, or 40, or60, or 80 ppmw of 3-CBA; less than about 1,000, or 800, or 500, or 300ppmw of 3-CBA; at least about 2, or 4, or 6, or 8 ppmw of 2,6-DCF; andless than about 120, or 80, or 60, or 40 ppmw of 2,6-DCF.

Another aspect of the present invention relates to improved dispositionof ambient water condensed and removed, rather than being retained asvapor, during compression of ambient air for oxidant supply. Compressionmeans for supplying pressurized ambient air often comprise multistagecompression systems using at least one intercooler to remove heat ofcompression to make the process more thermodynamically and mechanicallyefficacious. Such systems typically produce condensed water ininterstage coolers, and this water is typically separated from airbefore the inlet to a subsequent compression stage. Because suchoxidant-supply-intercooler-condensate may be contaminated withlubricants and/or seal fluids, said condensate is conventionallydirected to a wastewater treatment facility, perhaps only an oil skimmerthough sometimes comprising biological or other treatment technologies.

The inventors have discovered the following improved uses foroxidant-supply-intercooler-condensate, simultaneously reducingwastewater treatment costs and reducing costs for purchasing and/orpurifying utility water. It is preferred to feed at least a portion ofoxidant-supply-intercooler-condensate into a TPA process and/or adjacentPET process, more preferably in at least one of following process steps.A TPA solvent recovery and/or dehydration means, especially as reflux asdefined herein. A TPA filtrate solvent purification system, especiallyas an azeotropic separation aid for recovery or TPA catalyst components,more preferably comprising isopropyl acetate according to referencescontained herein. A process for purifying TPA by selective, catalytichydrogenation of crude TPA. A scrubber on an ambient vent from either aTPA or PET process or storage vessel.

It is preferred to feed at least a portion ofoxidant-supply-intercooler-condensate into at least one utility watersystem, more preferably comprising the following systems, and mostpreferably for use by a TPA process and/or adjacent PET process. Acooling water system, more preferably a cooling tower water systemwherein water is cooled by direct contact with air. A filtered watersystem. A de-ionized and/or de-mineralized water system. A fire watersystem.

The amount of oxidant-supply-intercooler-condensate varies with ambienthumidity, with cooling medium temperature, with pX feed rate, and withexcess dioxygen in reaction off-gas, both transiently and on average.Accordingly, it is preferred that preferred, disclosed uses comprise atleast about 0.01, or 0.02, or 0.04, or 0.08 kilogram ofoxidant-supply-intercooler-condensate per kilogram of pX fed tocorresponding oxidation reaction medium.

Generally, it is preferred to vent water vapor in approximatelycontinuous balance with its introduction via various process steps; butnonetheless, it is inevitable that there are production upsets andmaintenance activities that produce a temporary excess of liquidwastewater. For example, opening a process vessel for inspection orrepairs during a process shutdown is often preceded by thoroughly waterwashing or steaming to remove essentially all solvent, substrate, andproduct; and the amount of such upset-water can be large for majormaintenance activities. It is preferred to provide at least one storagereservoir for liquid wastewater and that this liquid wastewater issubsequently returned to at least one process step and subsequentlyvented as water vapor according to other disclosures herein. It ispreferred that such liquid wastewater storage reservoirs provideeffective isolation to control release of VOC to ambient surroundings.It is preferred that the volume of such storage reservoirs is at leastabout 50, or 100, or 200, or 400 cubic meters. It is preferred that thevolume of such storage reservoirs is less than about 12,000, or 9,000,or 6,000, or 3,000 cubic meters.

By combining various aspects relating to expelling water in vapor formto ambient surroundings, more preferably after treatment in a TOD, alongwith using oxidant-supply-intercooler-condensate and/or bromine scrubberwater in disclosed applications for process and/or utility water, it ispreferred to produce less than about 400, or 350, or 300, or 250, or200, or 100, or 50, or 20 gram of liquid wastewater effluent perkilogram of solid TPA product formed. It is preferred that thisperformance is maintained essentially continuously for long periods. Itis preferred to produce these low levels of liquid wastewater effluentfor at least about 60, or 80, or 90, or 95 percent of the time in acontinuous 24 hour period. It is preferred to produce these low levelsof liquid wastewater effluent averaged over a period of at least about4, or 16, or 64, or 254 days. A PET synthesis facility also produceswater from reactions converting at least a portion of TPA into PET, andthis water is often contaminated with various VOC compounds, e.g.,ethylene glycol, acetaldehyde, and various dioxolanes. It is preferredto treat at least a portion of contaminated water of PET formation in ashared, common facility along with water of TPA formation from anadjacent TPA facility. Preferably, said contaminated water from PETformation is left in vapor form exiting said PET facility for treatment,or it is converted to a vapor form using at least a portion of thermalenergy from said adjacent TPA facility.

In another embodiment of the invention, the amount of water generated asa byproduct or added to oxidation that is exiting said productionfacility to the ambient external environment as a vapor is at least 0.3,or 0.4, or 0.49 kilograms per kilogram of aromatic compound. As usedherein production facility can include wastewater treatment.

More preferably, at least a portion of vaporized water of PET formationfrom reactions in an adjacent PET synthesis facility is treated alongwith at least a portion vaporized water of TPA formation in a shared,common TOD, still more preferably an RTO, according at any and/or alldisclosures herein pertaining to the processing of reaction off-gas fromoxidation reaction medium. It is preferred that at least a portion ofoxidation reaction medium forming TPA is located such that the minimumhorizontal distance from said PET synthesis facility is less than about1800, or 900, or 300, or 100 meters. It is preferred that at least aportion of TPA is fed into a reaction medium of said adjacent PETsynthesis facility within less than about 72, or 24, or 12, or 4 hoursafter being formed from para-xylene. It is preferred to process at leastabout 40, or 60, or 70, or 80 weight percent of the off-gas from saidadjacent PET synthesis facility in a shared, common TOD, more preferablyRTO, with at least about 40, or 60, or 70, or 80 weight percent of thereaction off-gas from at least one primary oxidation reaction medium. Itis preferred to process at least about 40, or 60, or 70, or 80 weightpercent of the water of TPA formation from a TPA production facility isprocessed in a shared, common TOD, more preferably RTO, with at leastabout 40, or 60, or 70, or 80 weight percent of the water of forming PETin said adjacent PET production facility.

It is preferred that the normal flow of process wastewater effluent fromsaid adjacent PET production facility combined with the normal flow ofprocess wastewater effluent from said TPA production facility is lessthan about 400, or 200, or 100, or 50 grams of liquid wastewatereffluent per kilogram of PET formed. If the amount TPA produced in saidTPA production facility differs from the amount of TPA consumed in saidadjacent PET production facility by more than 10 weight percent, thenthe liquid wastewater effluent from said TPA facility is prorated by theratio of its usage in said PET facility, this value is summed with theliquid wastewater effluent from said PET facility, and the is divided bythe amount of PET produced.

We claim:
 1. A process for producing terephthalic acid and/orisophthalic acid, said process comprising: (a) oxidizing an aromaticcompound in at least one oxidizer to thereby produce an oxidizer off-gasand an oxidizer product comprising terephthalic acid and/or isophthalicacid, wherein the at least one oxidizer comprises a bubble columnreactor, and wherein the aromatic compound is at least one memberselected from para-xylene and meta-xylene; (b) introducing at least aportion of said oxidizer off-gas into a solvent recovery system tothereby produce a solvent-depleted off-gas and a recovered solvent; (c)adding hot combustion products to said solvent-depleted off-gas tothereby provide a heated off-gas comprising VOCs, wherein thetemperature of said heated off-gas is at least 10° C. greater than thetemperature of said solvent-depleted off-gas, wherein the amount ofthermal energy added to said solvent-depleted off-gas by said hotcombustion product is in a range from at least 100 watts per kilogram ofsaid aromatic compound fed to said oxidizer to less than 1,000 watts perkilogram of said aromatic compound fed to said oxidizer; and (d) passingat least a portion of said heated off-gas through a turboexpander,wherein at least 50 mole percent of the hydrocarbyl-compounds present insaid solvent-depleted off-gas exiting said solvent recovery system ispassed through said turboexpander.
 2. The process according to claim 1wherein said hot combustion products are added to said solvent-depletedoff-gas in an amount sufficient to maintain said heated off-gas at atemperature that is at least 5° C. above its local dew point temperatureat all points during passage of said heated off-gas through saidturboexpander.
 3. The process according to claim 1 further comprisingoxidizing a fuel with dioxygen to thereby produce said hot combustionproducts, where said fuel comprises hydrocarbyl compounds.
 4. Theprocess according to claim 3 wherein said fuel predominately comprisesmethanol, ethanol, methane, propane, butane, and/or fuel oil.
 5. Theprocess according to claim 3 wherein said fuel comprises at least 50weight percent methane.
 6. The process according to claim 3 furthercomprising compressing air in an air compressor to thereby providedcompressed air, further comprising introducing a first portion of saidcompressed air into said oxidizer, further comprising using a secondportion of said compressed air to supply at least a portion of saiddioxygen used to oxidize said fuel.
 7. The process according to claim 6wherein an oxidation catalyst is not used to promote said oxidizing ofsaid fuel.
 8. The process according to claim 7 wherein at least 10weight percent of the hydrocarbyl compounds present in saidsolvent-depleted off-gas are not combusted prior to exiting the laststage of said turboexpander.
 9. The process according to claim 8 whereinat least 50 percent of the methyl acetate present in saidsolvent-depleted off-gas removed from said solvent recovery system ispassed through said turboexpander.
 10. The process according to claim 1or 3 further comprising cooling a turboexpander off-gas exiting the laststage of said turboexpander in an off-gas condenser thereby condensingwater vapor present in said turboexpander off-gas to thereby provide acondenser effluent comprising a condenser off-gas and a condensedliquid.
 11. The process according to claim 10 further comprising passingat least a portion of said condenser effluent through a knock-out vesselto thereby separate said condenser effluent into a knock-out off-gas anda knock-out liquid.
 12. The process according to claim 11 furthercomprising subjecting at least a portion of said knock-out off-gas tothermal oxidative destruction (TOD) in a TOD device to thereby produceTOD off-gas.
 13. The process according to claim 12 further comprisingtreating said TOD off-gas in a bromine scrubber to thereby produce abromine-depleted off-gas.
 14. A process for producing terephthalic acid,said process comprising: (a) oxidizing para-xylene in at least oneoxidizer to thereby produce an oxidizer off-gas and an oxidizer productcomprising terephthalic acid, wherein the at least one oxidizercomprises a bubble column reactor; (b) introducing at least a portion ofsaid oxidizer off-gas into a solvent recovery system to thereby producea solvent-depleted off-gas and a recovered solvent; (c) adding hotcombustion products to said solvent-depleted off-gas to thereby providea heated off-gas comprising VOCs; and (d) passing at least a portion ofsaid heated off-gas through a turboexpander, wherein at least 50 molepercent of the hydrocarbyl-compounds present in said solvent-depletedoff-gas exiting said solvent recovery system is passed through saidturboexpander, wherein said hot combustion products are added to saidsolvent-depleted off-gas in an amount sufficient to maintain said heatedoff-gas at a temperature that is at least 5° C. above its local dewpoint temperature at all points during passage of said heated off-gasthrough said turboexpander, and wherein the temperature of said heatedoff-gas is at least 10° C. greater than the temperature of saidsolvent-depleted off-gas; wherein the amount of thermal energy added tosaid solvent-depleted off-gas by said hot combustion product is in arange of 100-1000 watts per kilogram of said aromatic compound fed tosaid oxidizer.
 15. The process according to claim 14 further comprisingoxidizing a fuel with dioxygen to thereby produce said hot combustionproducts, where said fuel comprises hydrocarbyl compounds.
 16. Theprocess according to claim 15 wherein said fuel predominately comprisesmethanol, ethanol, methane, propane, butane, and/or fuel oil.
 17. Theprocess according to claim 15 wherein said fuel comprises at least 50weight percent methane.
 18. The process according to claim 15 furthercomprising compressing air in an air compressor to thereby providedcompressed air, further comprising introducing a first portion of saidcompressed air into said oxidizer, further comprising using a secondportion of said compressed air to supply at least a portion of saiddioxygen used to oxidize said fuel.
 19. The process according to claim18 wherein an oxidation catalyst is not used to promote said oxidizingof said fuel.
 20. The process according to claim 19 wherein at least 10weight percent of the hydrocarbyl compounds present in saidsolvent-depleted off-gas are not combusted prior to exiting the laststage of said turboexpander.
 21. The process according to claim 20wherein at least 50 percent of the methyl acetate present in saidsolvent-depleted off-gas removed from said solvent recovery system ispassed through said turboexpander.